Voltage multipliers or cascade rectifiers are circuits which comprise diodes and so-called push-pull capacitors that are charged by an AC current via the diodes. The diodes and capacitors are arranged in multiplier stages in such a way that the voltage from a positive peak of the AC current is added to the voltage of a negative peak in each stage and the output voltage is doubled in each stage.
High voltage rectifiers for medical x-ray imaging devices frequently comprise a high voltage multiplier which multiplies the output voltage of a high voltage high frequency transformer to reach a required high output voltage of, for example, up to 160 kV. A disadvantage of those circuits may be that the root-mean-square-value of the pulse-like shaped current in the diodes is substantially higher than the average current, which is responsible for the output power and thus the losses in the diodes are significantly increased.
Another disadvantage of these circuits may be that the total current in the push-pull capacitor increase towards the transformer which may generate a high stress in the transformer-side push-pull capacitors. This may prevent the usage of small size ceramic capacitors for the push-pull capacitors.
In WO 2007/017793 A1 a proposal was made how to mitigate the pulse-shaped current in the diodes by adding additional equalizing capacitors. The additional equalizing capacitors are not subject to a DC bias and thus may be very small and cheap. With the design of WO 2007/017793 A1 the push-pull capacitor must be charged via the diodes of the last stage and the center tap of the transformer since all other DC current paths are now blocked by capacitors.
Another proposal avoids push-pull capacitors of the first stage by not grounding the transformer and leaving it in floating operation at an average of half of the voltage of the first multiplier stage. This allows a removal of the first two push-pull capacitors and thus removal of some relevant part of the losses.